The present invention relates generally to high-speed data transmission and, more particularly, to a method for adaptively reducing linear and nonlinear distortion in a digital communications channel.
During the past thirty years, there have been extensive efforts put forth seeking new techniques for combatting the intersymbol interference (ISI) in data transmission over band-limited channels. Adaptive equalization has been the major technique that has allowed a substantial increase in attainable transmission rate. Linear fractionally spaced equalizers have been used in systems having phase distortion in the channel to eliminate virtually all ISI without increasing the noise level. When amplitude distortion is present in the channel, however, adaptive linear equalizers inevitably provide noise enhancement. Decision-feedback equalization has been shown to offer somewhat improved performance when amplitude distortion is present.
In a typical digital communications system the transmitted data elements assume values from an alphabet of symbols. In such a system, irrespective of the modulation format being used, each received data sample, referred to as the observable, following the demodulation process, comprises a composite of the transmitted data symbol which has been affected by frequency independent channel attenuation or gain, intersymbol interference that may have been generated by a variety of system imperfections, and Gaussian thermal noise. In general, the value of ISI at each sampling time will depend on the overall system characteristic, i.e., its impulse response, and on a finite length data sequence which includes the observable as one of its elements, and whose length is determined by the memory of the system. Assuming a linear system, the dependency of the ISI on the elements of this data sequence is a linear one. However, for the nonlinear system, this relationship becomes nonlinear.
In a process for cancelling the system-induced distortion, the objective is to add a noiseless cancellation term to the observable which does not enhance the variance of thermal noise already present in the received waveform. Distortion cancellation has been disclosed in the prior art for linear and nonlinear systems using various canceller embodiments. In each such disclosure, a preliminary estimate of the data sequence is used to generate the value of the cancellation term. This preliminary estimate of the sequence, however, is used blindly and is not questioned in any way regarding the possibility of containing erroneous components. Clearly, since the cancellation term is dependent on the system characteristic and on the data sequence, errors in the estimations of this sequence will result in an estimated distortion cancellation term which will not, in general, be equal to the desired value. Therefore, given that errors have been made in estimating the data sequence, there will exist a cancellation term error with a magnitude depending, in general, on the number of errors made and on the system characteristic prevailing at the time the errors were made. Thus, it is seen that the performance of a canceller as disclosed in the prior art will degrade.